DESCRIPTION: Making emergency contraceptives widely available is the single most important step that could be taken to reduce the unacceptably high number of unintended pregnancies and induced abortions in the United States. The goal of this project is to estimate the effectiveness of the Yuzpe regimen of emergency postcoital contraception, one of the three forms of emergency contraception, one of the three forms of emergency contraception available in the United States. The Yuzpe method involves taking two doses of combined estrogen/progestin pills, with each dose containing 100 ug of ethinyl estradiol (EE) and 1.0 mg of dl-norgestrel. One dose is taken within 72 hours - preferably as soon as possible - after unprotected coitus, and the other is taken 12 hours later. Documenting accurately the efficacy of the Yuzpe method is very important not only to help women make informed choices but also in gaining approval from the Food and Drug Administration for the use of oral contraceptives as emergency contraception. Without FDA approval, manufacturers cannot market or advertise these products for postcoital use, so women will remain unaware of this option. The investigators review in depth all published results from clinical trials of the Yuzpe method in which data were collected and reported on the cycle days on which unprotected intercourse occurred. The effectiveness rate is defined as the proportionate reduction in the probability of conception caused by use of emergency contraception. In order to compute the effectiveness rate, the investigators need information not only on the observed number of pregnancies but also on the expected number of pregnancies. For this reason they review only those trials that such information was provided in or could be derived from the published reports. The expected number of pregnancies can be computed from external estimates of the risk of conception for each day of the cycle provided that information on the cycle day of intercourse relative to the cycle day of ovulation is available for subjects in the trial. All current estimates of the effectiveness of the Yuzpe method are derived from estimates of conception probabilities by cycle day provided by Dixon et al. [1980]. As argued in this application, these estimates are biased downward, so that the effectiveness of the Yuzpe method is also biased downward. The investigators propose to re-estimate the effectiveness rate by using new estimates of conception probabilities specific to each cycle day that do not suffer from such bias.